The surface structure of at least approximately tubular inner surfaces of a cavity can be captured three-dimensionally by means of optical triangulation. In this process a spatially structured illumination pattern is projected onto the inner surface of the respective cavity to be captured and the scene is captured digitally by means of a camera system. By measuring the distortion of the illumination pattern projected onto the inner wall as a result of the surface shape, which can be done automatically using known image processing methods, it is possible to calculate a digital model, which maps the shape of the cavity. Deviations and/or distortions of the captured projected lines from the known, initially symmetrical circular shapes that were concentric to an optical axis are captured in this process.
Such a cavity measurement by means of optical triangulation can advantageously be used when measuring or profiling the human auditory canal. The anatomy of the auditory canal means that an optical measuring device must be provided, which cannot exceed a maximum diameter of 4 mm. This basic condition applies to the entire object-side optical system of such a measuring device, said optical system having to be inserted into the auditory canal. The object-side optical system here comprises at least a camera system and an optical element for producing the structured illumination. The camera system and the optical element are disposed concentrically to a common optical axis of the optical measuring device here.
It is known that diffractive optical elements can be used to produce structured illumination. In particular (binary) phase gratings, also known as so-called Dammann gratings, can distribute the incident intensity of a primary light beam bundle selectively and in some instances largely uniformly to specific orders of diffraction due to a particularly advantageous substructure.
A so-called circular Dammann grating for producing a structured illumination pattern from concentric rings is known from the publication “Changhe Zhou, Jia Jia, Liren Liu; Circular Dammann Grating; Optics Letters, Vol. 28, No. 22, 2003, pages 2174-2176”. However this has the disadvantage that it is difficult to achieve larger deflection angles in relation to the optical axis of the circular phase grating. It is true that larger deflection angles would in principle be possible using extremely small phase grating structures in the region of 150 mm but such small phase grating structures are technologically extremely difficult to produce. To produce such fine gratings, etching processes are needed which require much finer structuring than the etching processes currently used with a best resolution of 400 nm.